New H2 collision-induced absorption and NH3 opacity and the spectra of the coolest brown dwarfs

We present new cloudy and cloudless model atmospheres for brown dwarfs using recent ab initio calculations of the line list of ammonia (NH3) and of the collision-induced absorption of molecular hydrogen (H2). We compare the new synthetic spectra with models based on an earlier description of the H2 and NH3 opacities. We find a significant improvement in fitting the nearly complete spectral energy distribution of the T7p dwarf Gliese 570D and in near infrared color-magnitude diagrams of field brown dwarfs. We apply these new models to the identification of NH3 absorption in the H band peak of very late T dwarfs and the new Y dwarfs and discuss the observed trend in the NH3-H spectral index. The new NH3 line list also allows a detailed study of the medium resolution spectrum of the T9/T10 dwarf UGPS J072227.51-054031.2 where we identify several specific features caused by NH3.Comment: 37 pages, 13 figures. Accepted for publication in the Astrophysical

Theory of collision-induced translation-rotation spectra: H2-He

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.29.595.An adiabatic quantal theory of spectral line shapes in collision-induced absorption and emission is presented which incorporates the induced translation-rotation and translation-vibration spectra. The generalization to account for the anisotropy of the scattering potential is given. Calculations are carried out of the collision-induced absorption spectra of He in collisions with H2 with ab initio electric dipole functions and realistic potentials. The anisotropy of the interaction potential is small and is not included in the calculations. The predicted spectra are in satisfactory agreement with experimental data though some deviations occur which may be significant. The rotational line shapes have exponential wings and are not Lorentzian. The connection between the quantal and classical theories is written out explicitly for the isotropic overlap induction

Comparison of the Calculated Collision-Induced Absorption Spectra by Dense Hydrogen-Helium, Deuterium-Helium, and Tritium-Helium Gas Mixtures

We have recently determined the induced dipole surface (IDS) and potential energy surface (PES) of collisional H2-He complexes. We have used these surfaces to compute the binary collision-induced absorption spectra of H2 molecules interacting with He atoms and of D2 molecules interacting with He atoms. Here we extend these calculations to the case of T2 molecules interacting with He atoms. Whereas the electronic structure of X2-He is virtually the same for all hydrogen isotopes X = H, D, or T, the collisional dynamics and molecular scattering wave functions are different for the different collisional pairs. We have calculated spectra up to a temperature of 9000 K and frequencies up to 20,000 cm−1. Here we compare the calculated collision-induced absorption spectra for the different hydrogen isotopes. While we have observed reasonable agreement between our calculations and laboratory measurements for the collisional H2-He and D2-He complexes, there are no laboratory measurements for T2-He collisional complexes, and one must rely on the fundamental theory, supported by the agreement between theory and experiment for the other isotopes.</jats:p

Collision-induced Absorption in Gases

This book reviews the present knowledge of collision-induced absorption of infrared radiation in the dense, common gases. Following a brief introduction and review of essential background information, such as dipole radiation, molecular collisions and interactions, numerous experimental results for the absorption spectra of dense gases are presented. Other chapters review the causes and properties of dipole moments induced by molecular interactions, the theory of collision-induced absorption in monatomic gas mixtures and in molecular gases and mixtures. The final chapter discusses related phenomena and the important applications in astrophysics. Throughout the book, the emphasis is on the absorption by binary molecular complexes, but the onset of many-body effects, such as the ternary contributions and the intercollisional process, are also considered. The volume is meant to be a practical guide and sourcebook for the researcher interested in the spectroscopy of dense, neutral fluids. This edition includes a new appendix reviewing recent work.</jats:p

Is Sonoluminescence due to Collision-Induced Emission?

We estimate the collision-induced emission (CIE) intensity and profile in the visible and near UV region of the spectrum of N2-X pairs, where X represents another N2molecule or an argon atom, etc. of shock waves believed to exist in sonoluminescence experiments. Calculated profiles consist of superimposed high overtone bands and resemble the measured profiles. Intensities calculated on the basis of a few, simple assumptions concerning the induced dipole surface compare favorably with measurements. The agreement obtained suggests that CIE is an attractive alternative to bremsstrahlung to explain sonoluminescence. The CIE source is optically thin, and the spectral emission profile is not related to Planck's law.University of TexasApplied Research Laboratorie